This invention relates generally to gas detector units for monitoring noxious gases in the environment in the area where the units are placed so that persons involved in the environment where the instruments are located may be warned of the presence of a too high level of a noxious gas. More particularly, this invention relates to such a gas sensing unit which has incorporated therein provisions for recalibration of the instrument automatically at periodic intervals without the need for specific recalibration by an individual. That is, the instrument is controlled by a connected microprocessor-type instrument which controls the periodic recalibration of the gas sensing unit. By proper control through the microprocessor, various valve connections may be operated to cause the gas sensing unit to separate from its usual monitoring function and to calibrate through specific procedures, so as to accommodate changes in the instrument over a period of time and changes in the environment where it is functioning. The recalibration is incorporated into the microprocessor so that subsequent monitoring readings are automatically adjusted to reflect the recalibration. This invention is an improvement over the inventions described in U.S. Pat. Nos. 3,992,267, issued Nov. 16, 1976; 3,824,167, issued July 16, 1974; 3,776,832 issued Dec. 4, 1973 and 3,909,386, issued Sept. 30, 1975. Each of these patents is incorporated by reference in its entirety herein.
With ever increasing concern about pollution of our environment and our increasingly sophisticated knowledge with respect to the presence of polluting materials in the environment, attempts have been made to develop systems which will protect us by warning of increases in the concentration of certain substances in the ambient to a level which is toxic and/or otherwise dangerous to our existence. One such device which has been developed in recent years is a gas sensor operated through an electrochemical cell for sensing the presence of such gases adjacent a work area such as, for example, a mine shaft. As will be appreciated, it is important that such sensors continue to operate over a period of time so that certain enclosed areas are protected where certain levels of concentration of gases may cause death if exposure is for a specific period of time. It is important, also, from a manpower standpoint that the instruments need not be continuously attended to because of any rapid deterioration of the sensing capacity thereof. However, electrochemical gas sensors, for example, are subject to certain limitations over a period of time merely because of the chemical nature in which they operate, in the sense that the sensing capacity changes with environmental conditions.
Thus, there is a need with respect to present day instruments for maintenance at periodic intervals, and especially recalibration thereof. That is, such instruments change their performance characteristics over a period of time and it is necessary that they be recalibrated so as to restore accuracy. For example, presently carbon monoxide monitors which are placed around a hazardous area such as a blast furnace for monitoring, for example, 100-600 ppm CO are being calibrated once a month by a maintenance routine which requires an operator to visit each instrument installed for this purpose. The operator carries a certified calibration gas mixture of carbon monoxide and air which is typically 250 parts per million carbon monoxide/air. The certified span gas mixture is injected into the instrument to verify its response, and an adjustment is made in the instrument sensitivity, if required.